Unshored Excavation Beside a Neighbour's Boundary Wall
An excavation for an isolated footing is 1.8 m deep and 2.4 m × 2.4 m in plan, dug by four labourers, with one face only 1.2 m from a neighbour's 230 mm brick boundary wall that is 2.4 m high. The soil is loose silty fill to 1.5 m, then stiff clay. It has rained for two days and water is seeping in at about 1.2 m depth. Spoil is heaped 0.4 m from the pit edge to a height of 1.0 m. Workers climb in and out along the cut face; there is no barricade, and the site opens onto a lane used by schoolchildren. The contractor wants the footing concreted before the weekend and has told the gang to 'work fast and not worry'.
1. Identify the five most serious hazards, ranked by likelihood × severity, with your reasoning.
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- Collapse of the pit face next to the boundary wall. The soil is loose silty fill, saturated by two days of rain, and the face is only 1.2 m from a 2.4 m wall. Likelihood is high and severity is fatal - a worker buried, and the wall could fall into the lane. This ranks first.
- Undermining and collapse of the boundary wall itself. Digging 1.8 m down within 1.2 m of a wall almost certainly cuts into the zone of influence of its foundation. Collapse onto the lane or onto workers is very serious.
- A child or passer-by falling into the open pit. There is no barricade and the lane is used by schoolchildren; likelihood is moderate but severity is high.
- Surcharge from the spoil heap. Spoil at 0.4 m from the edge, 1.0 m high, adds load exactly where the face is weakest and raises collapse likelihood.
- Unsafe access and water. Climbing the cut face causes falls and further erodes the face; seepage at 1.2 m softens the toe and creates a drowning/slipping hazard. Ranked fifth only because the others are more likely to kill.
2. Specify the shoring or sloping and dewatering measures you would require before anyone re-enters. Give dimensions.
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Stop work and clear the pit first.
Shoring. At 1.8 m in loose fill, shoring or safe sloping is mandatory. Because the wall is only 1.2 m away, battering is not acceptable on that face. Use close-sheeted shoring - timber or steel sheets driven to below pit level, with walers and adjustable struts at not more than 1.5 m vertical spacing - along the wall face and at least the two adjacent faces. Provide a 1.0 m minimum clear berm between the pit edge and any material.
Spoil. Move the spoil at least 1.0 m from the edge and keep the heap not more than 0.5 m high.
Water. Cut a sump 0.6 m × 0.6 m × 0.6 m at one corner, dewater with a pump, and discharge well away from the pit so it does not return. Do not allow water to pond at the edge.
Access and edge protection. Provide a ladder extending 1 m above the top, secured, and a rigid barricade 1.2 m high around the pit with warning lights at night.
Wall. Get the wall inspected by a competent person; if cracking appears, underpin or prop it before further digging.
3. The contractor says a 1.8 m pit is 'shallow and does not need any paperwork'. Respond with the management controls, including what you would record and whom you would inform.
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Depth alone does not decide this. The controls are driven by the soil, the water, the adjacent wall and the public interface, all of which are adverse here. The correct response:
- Written risk assessment and method statement before entry, covering shoring, dewatering, access and rescue.
- Excavation permit signed by a competent supervisor, plus a daily inspection before each shift and after every rain, recorded in a register.
- Condition survey of the neighbour's wall with dated photographs before work, and written notice to the neighbour.
- Barricading and diversion of the lane, or a banksman, given the schoolchildren.
- PPE - helmets, boots, gloves - and a standby person at the top.
- Records: depth, soil description, shoring type, water level, inspection dates, names of workers, incidents.
- Notify the principal employer/client in writing, and the local authority if the wall shows distress; keep accident records as required under the Building and Other Construction Workers Act, 1996 and the state rules.
'Work fast and not worry' is not an instruction that can be followed. The supervisor should refuse entry until these controls exist, and record the refusal.